Hi,
A majority of premature hair greying lies in the inability to make or transfer pigment from the skin cells to the hair shaft. Genetics play a major role. Stress, smoking and vitamin deficiencies have also been implicated as aggravating factors but their roles are less certain compared to genetic factors.
In a small group of patients with localized white hairs due to an autoimmune process such as vitiligo or alopecia areata, the white hairs are reversible once the underlying condition has been treated.
However, for a majority of patients with genetic hair greying, there is currently no scientifically proven treatment or method to slow it down or reverse it.
Hope this helps!